Abstract
It was recently discovered that condensation growing on a nanostructured superhydrophobic
surface can spontaneously jump off the surface, triggered by naturally
occurring coalescence events. Many reports have observed that droplets must grow to
a size of order 10 μm before jumping is enabled upon coalescence; however, it remains
unknown how the critical jumping size relates to the topography of the underlying
nanostructure. Here, we characterize the dynamic behavior of condensation growing
on six di↵erent superhydrophobic nanostructures, where the topography of the
nanopillars was systematically varied. The critical jumping diameter was observed to
be highly dependent upon the height, diameter, and pitch of the nanopillars: tall and
slender nanopillars promoted 2 μm jumping droplets while short and stout nanopillars
increased the critical size to over 20 μm. The topology of each surface is successfully
correlated to the critical jumping diameter by constructing an energetic model that
predicts how large a nucleating embryo needs to grow before it can inflate into the air
with an apparent contact angle large enough for jumping. By extending our model
to consider any possible surface, it is revealed that properly designed nanostructures
should enable nanometric jumping droplets, which would further enhance jumpingdroplet
condensers for heat transfer, anti-fogging, and anti-frosting applications.
